In conventional AM and FM audio transmission there is a substantial amount of background noise. For this reason, signals at a very low amplitude (quiet signals) are amplified so that they are substantially above the residual noise level of the transmission and are thus audible at a receiver. If this amplification were applied to the whole signal then the amplitude of relatively high amplitude (loud) signals would be unacceptably high. Thus the amplification (gain) has to be reduced in the loud portion, i.e. the dynamic range of the whole signal has to compressed by raising the level of quiet signals and reducing the level of loud signals. Not all broadcast programmes have a wide dynamic range and it is often considered satisfactory to permit the limiters which prevent over-modulation of transmitters to compress the programme dynamics by controlling peaks. Sometimes proprietory compression equipment is employed prior to the limiter. However, these devices have an obtrusive effect on "serious" music. Dynamic range compression of serious music is usually done by a studio manager who is, for example, familiar with the piece of music which is to have its dynamic range compressed and can make unobtrusive adjustments. With current broadcasting standards a hi-fi enthusiast cannot receive the full dynamic range of a piece played on high quality source materials at the studio, such as compact disc, because of the limitations of the broadcast medium, necessitating a degree of compression. This represents a compromise between different listeners' requirements. Sometimes the degree of compression is varied during the day in dependence on whether the majority of the audience are expected to be at home or in their cars. Whatever approach is adopted it is not possible to satisfy all listeners all of the time.
Digital audio broadcasts will have an ancilliary data channel associated with them and it has been proposed that this could carry dynamic range or compression related information. Thus it would be possible for receivers to be equipped with circuitry which could adapt the dynamic range of the signal to the listeners requirements. For example, a signal transmitted with full dynamic range could be transmitted together with a compression control signal with which the receiver could, if so instructed by the listener, compress the dynamic range by up to, say 20 dB. Alternatively a compressed signal could be broadcast and listeners with hi-fi receivers could set their receivers to re-expand the signal dynamics by part or all of the compression. The compression control signal could be sent with a fixed rate of,for example, 24 bits per second.